At most casinos, you get one point for every dollar wagered on a slot machine (or video poker). I have seen casinos that require you to wager up to $2.50 to get a point though, so it can vary.
That does not mean you must SPEND $25, just wager $25. Unless you have to worst luck on the planet, it shouldn't take more than $15 or so, and of course you could win some money. But really, if you plan on playing slots or video poker at all, 25 points is no big deal. You'll earn that without trying.
Slot tournaments are really simple. You basically just show up before it starts, and they sit you down at one of a group of designated tournament machines. They say "go!" and you just mash the button until time is up. Whomever has the most credits on their machine at the end wins.
Good luck and have fun.